1. Field of the Invention
The invention relates to an evaporation source, in particular for use in a sputtering process or in a vacuum evaporation process, preferably a cathode vacuum evaporation process.
2. Discussion of Background Information
A whole series of different chemical, mechanical and physical techniques are known from the prior art for the application of layers or layer systems on the most varied substrates, each of which techniques are valid and have corresponding advantages and disadvantages in dependence on the demand and on the area of use.
Processes are in particular common for the application of comparatively thin layers or films in which the surface of a target is changed into the vapor form in an arc or atoms from a surface of a target are changed into the vapor form by means of ionized particles, with the vapor thus formed then being able to be deposited on a substrate as a coating.
In a conventional embodiment of cathode atomization, the target is connected in a sputtering process to a negative non-pulsed DC voltage source or e.g. to a unipolar or bipolar pulsed source or is operated in HIPIMs mode or is connected to a radio frequency power source. The discharge is as a rule amplified by magnet systems. Magnetron atomization is then spoken of. The substrate is the material to be coated and it is located opposite the target, for example. The substrate can be grounded, floating, biased, heated, cooled or can be subjected to a combination thereof. A process gas is introduced into the process chamber, which inter alia contains the process electrodes and the substrate, to create a gas atmosphere in which a glow discharge is triggered and can be maintained. The gas pressures range in dependence on the application from a few tenths of a Pascal to several Pascals. Argon is a frequently used atomizer gas.
When the glow discharge is triggered, positive ions impact the surface of the target and predominantly release neutral target atoms by impact force transmission. They condense on the substrate to form thin films. There are additionally other particles and radiations which are generated by the target and all have film-forming properties (secondary electrons and ions, desorbed gases and photons). The electrons and negative ions are accelerated toward the substrate platform and bombard it and the growing film. In some cases, a bias potential is applied to the substrate holder, for example, so that the growing film is exposed to the bombardment with positive ions. This process is also known as bias atomization or ion plating.
In certain cases, not only argon is used, but also other gases or gas mixtures. This typically includes some types of reaction atomizer processes in which a composition is synthesized by atomization of a metal target (e.g. B, Ti) in an at least partly reactive reaction gas to form a composition of the metal and the reaction gas types (e.g. titanium oxides). The atomization yield is defined as the number of atoms expelled from the target surface per indecent ion. It is an essential parameter for characterizing the atomizer process.
An estimated one percent of the energy incident on a target surface typically results in the expulsion of atomized particles; 75% of the incident energy results in a heating of the target and the remainder is dissipated by secondary electrons, for example, which can bombard and heat the substrate. An improved process known as magnetron atomization uses magnetic fields for conducting the electrons away from the substrate surface, whereby the heat effect is reduced.
For a given target material, the application rate and the uniformity are influenced inter alia by the system geometry, the target voltage, the atomizer gas, the gas pressure and the electrical power applied to the process electrodes.
One used physical coating process is the known arc evaporation in its many embodiments.
In arc evaporation, the target material is evaporated by the effect of vacuum arcs. The target source material is the cathode in the arc circuit. The base components of a known arc evaporation system include a vacuum chamber, a cathode and an arc current connection, parts for igniting an arc on the cathode surface, an anode, a substrate and a power connection for a substrate bias voltage. The arcs are maintained by voltages in the range from 15-50 volts depending on the target cathode material which is used. Typical arc currents lie in the range from 30-400 A. The arc ignition takes place by the customary ignition processes known to the skilled person.
The evaporation of the target material from the cathode which forms the target is produced as the result of a cathode arc spot which in the simplest case is moved without regulation on the cathode surface at speeds of typically 10 m/s. The arc spot movement can in this respect, however, also be controlled with the aid of suitable confinement limits and/or magnetic fields. The target cathode material can be a metal or a metal alloy, for example.
The arc coating process is considerably different from other physical vapor coating processes. The core of the known arc processes is the arc spot which generates a material plasma. A high percentage, e.g. 30%-100%, of the material evaporated by the cathode surface is ionized as a rule, with the ions being able to exist in different charge states in the plasma, for example as Ti+, Ti2+, Ti3+, etc. The kinetic energy of the ions can in this respect move in the range from e.g. 10-100 eV.
These features result in coatings which can be of a very high quality and can have specific advantages in comparison with those coatings which are applied by other physical vapor coating processes.
The layers applied using arc evaporation usually show a high quality over a wide range of coating conditions. Stoichiometric compound films, for example, having very high adhesion and a high density can thus be manufactured which deliver high coating quantities for metals, alloys and compositions having excellent coating uniformity over a wide range of the reaction gas pressure. A further advantage among others is also the comparatively low substrate temperatures and the relatively simple manufacture of compound films.
The cathode arc results in a plasma discharge within the material vapor released from the cathode surface. The arc spot is normally some micrometers large and has current densities of 10 amperes per square micrometer. This high current density causes a lightning fast evaporation of the starting material. The vapor generated is composed of electrons, ions, neutral vapor atoms and microdroplets. The electrons are accelerated toward the clouds of positive ions. The emissions of the cathode light spot are relatively constant over a wide range of the arc current when the cathode spot is divided into a plurality of dots. The average current per dot depends on the nature of the cathode material.
In this respect, the geometry of the evaporation source also often plays a substantial role. It is thus known to use both rectangular and cylindrical evaporation sources. The selection of the geometry of the evaporation source can in this respect depend on a plurality of parameters and marginal conditions of the specific coating application.
A disadvantage of the coating plant and of the process in accordance with WO 90/02216 is inter alia that a uniform quality of the coatings is in particular not ensured. The quality of the applied layers thus varies as the consumption of the cathodes increases if the method parameters are not tracked in a complex and/or expensive manner. This is as known inter alia due to the fact that the rectangular cathodes are consumed in a non-uniform manner so that, with the same process parameters, the quality of the coating vapor becomes increasingly worse as the erosion of the cathodes increases because e.g. disturbing droplets form to an increasing degree in the arc evaporation, which has a negative effect on the layers. To keep these negative effects within limits, the cathodes have to be replaced prematurely, which is correspondingly expensive and complicated.
A further disadvantage in addition to the irregular erosion of the cathodes is that a control of the arc on the cathode is very difficult and complicated, if possible at all.
It is therefore frequently more advantageous to use a cylindrical cathode, which has the advantage, as is known, that the cylindrical cathode is rotatable about its cylinder axis and the consumption of the target material can thus be handled better. These problems have already been discussed in detail before in EP 2 159 821. By using evaporation sources in accordance with EP 2 159 821, the quality of the applied layers does not change with increasing consumption of the cathodes and the method parameters do not have to be tracked in a complex and/or expensive manner. This is inter alia due to the fact that the cylindrical cathodes are consumed uniformly so that, with the same process parameters, the quality of the coating vapor remains the same as the erosion of the cathodes increases and therefore does not become worse because e.g. disturbing droplets form to an increasing degree in the arc evaporation, which has a negative effect on the layers. Since these negative effects practically no longer occur with evaporation sources in accordance with EP 2 159 821, the cathodes no longer have to be exchanged prematurely, which results in corresponding significant cost savings.
A problem of cylindrical cathodes which has not yet been really satisfactorily solved is, however, their cooling. As initially mentioned, a large portion of the energy introduced into the evaporation sources is converted into heat which naturally has to be conducted away from the evaporation source again. A number of proposals are known for this purpose from the prior art.
DD 217 964 A3, for example, describes a cylindrical evaporation source in which the cooling is essentially conducted in proximity with the cylinder axis of the evaporation source. EP 1 173 629 B1 shows a similar solution. A substantial disadvantage of such solutions is that the cooling takes up a significant space in the interior of the evaporation source which is then not available for further necessary installations such as for magnet systems which the skilled person would often advantageously like to accommodate in the interior of the evaporation source. It was in addition shown that the conducting away of the heat is frequently not sufficiently efficient in such cooling systems, the installations are complicated and thus require frequent maintenance, which all in all naturally also drives up costs.